This invention relates to a process for preparing crosslinked polyaspartate salt.
Crosslinked polyaspartate salt (PAS) has an important industrial superabsorbing property making it useful in applications such as thickeners, flocculants, absorbers and the like. As noted in U.S. Pat. No. 5,461,085 to Nagatomo et al, for effective superabsorption of many times its own weight, the polysuccinimide (PSI) precursor for the salt should have a molecular weight (Mw) of at least 20,000. PAS is prepared by hydrolyzing crosslinked PSI with aqueous base. In conducting such hydrolysis, the high molecular weight, crosslinked PAS particles absorb whatever water is capable of being held to create a gel. To insure that the gel is well distributed for thorough, efficient hydrolysis reaction of uncrosslinked succinimide rings with base, it was conventional in the prior art to use a large excess of water. Hydrolysis was considered complete when the gel could not consume further base at pH 9-11, 60.degree. C. Note the Examples of U.S. Pat. No. 5,525,682 to Nagatomo et al where (col. 5, line 23) the polymer concentration during hydrolysis is 0.5 to 10 wt. % This water must eventually be removed, typically by evaporation, to form dry, crosslinked PAS since it will likely be used in this form in commercial applications--e.g. between layers of a diaper. It would be desirable to simplify the process of preparing and recovering such high molecular weight, crosslinked polyaspartate salt.